The present invention relates generally to augmented turbofan engines and, more particularly, to means and method for reducing differential pressure loading across an afterburner liner in an augmented turbofan engine.
A conventional augmented turbofan engine includes a fan which provides a portion of fan air to a core engine for generating combustion exhaust qases. Surrounding the core engine is a bypass duct which receives another portion of the fan air which bypasses the core engine. An afterburner or augmentor is disposed downstream of the core engine and bypass duct and includes a combustion liner within which is received the core engine exhaust gases and a portion of the bypass duct airflow, which is mixed with fuel for combustion in the afterburner. The afterburner also includes an annular plenum surrounding the liner, which receives the remaining portion of the bypass duct airflow for cooling the afterburner, in particular, the liner thereof.
A mixer is disposed at a downstream end of the core engine for mixing the bypass airflow with the core engine exhaust gases at the downstream end, which defines a match plane where the bypass airflow and the core engine exhaust gases intersect.
These engines have a match plane pressure ratio defined as the ratio of bypass airflow total pressure to core engine exhaust gas static pressure at the match plane. Match plane pressure ratios vary over a range during the operation of an engine from minimum to maximum values. In an exemplary augmented turbofan engine, the match plane pressure ratio varies from a minimum value of about 1.04 to a maximum value of about 2.8 during engine operation. During engine reheat operation, i.e. during operation of the afterburner, the match plane pressure ratio has values within the range of about 1.04 to 1.5 for the exemplary engine. During dry or non-reheat operation of the engine, i.e. when the afterburner is not in operation, the match plane pressure ratio may approach 2.8.
Since the match plane pressure ratio is a correlation between pressure of the bypass airflow and Pressure of the core engine exhaust gas, it is also an indication of the differential pressure loading occurring across the afterburner liner. More specifically, the differential pressure loading occurs since the bypass airflow is channeled to the plenum on the radially outer surface of the liner and the core engine exhaust gases are channeled in the augmentor and are bounded by the radially inner surface of the liner.
In the exemplary augmented turbofan engine, the bypass airflow is channeled directly to the augmentor plenum without incurring any significant pressure loss. Pressure losses are undesirable, since they decrease the aerodynamic efficiency of the engine. However, in such an engine, differential pressure loading across the liner due to the difference in pressure between the bypass airflow in the plenum and the pressure of the core engine exhaust gases within the liner increases as match plane pressure ratios increase. Accordingly, maximum differential pressure acting across the liner occurs at the maximum match plane pressure ratio, which occurs in the engine dry operation.
A differential pressure loading is a buckling load for the liner, and for some augmented turbofan engine designs that load may be substantial and therefore requires appropriate structure for accommodating the load.